Method for continuous production of electrolytic manganese dioxide

ABSTRACT

Method for continuous production of electrolytic manganese dioxide by electrolysis with an aqueous solution of manganese sulfate-sulfuric acid containing as impurity 30 g/l or more of magnesium in the form of magnesium sulfate while making up for consumed manganese by addition of a neutral aqueous manganese sulfate solution to electrolyte, is improved by concentrating said neutral aqueous manganese sulfate solution under reduced pressure at 40*C - 60*C to the volume corresponding to saturation point at from 15*C to 60*C and then cooling it to 15*C - 25*C. Crystal thus precipitated which mainly consists of magnesium sulfate is filtered away and the filtrate is added to the electrolyte to compensate for the consumed manganese. During electrolysis the concentration of magnesium in the form of magnesium sulfate is maintained at 45 g/l or less for avoiding troubles caused by the accumulation of magnesium sulfate in the electrolyte.

United States Patent Era et al.

[ 1 Aug. 19, 1975 MANGANESE DIOXIDE [75] Inventors: Akio Era; Takeo Emoto, both of Tokyo. Japan {73] Assignee: Mitsui Mining & Smelting Cu., Ltd.,

Tokyo, Japan {22] Filed: Dec. 1], I973 [21] Appl. No.: 423,760

[30] Foreign Application Priority Data Sept. 20, I973 Japan 48406182 [52] U.S. Cl 204/96; 423/158 I] Int. Cl o. COII) 45/02 [58] Field of Search 204/96; 423/l58 [56] References Cited OTHER PUBLICATIONS "Soluhilities of Inorganic & Metal Organic Compounds," by Seidell. Vol l, 1940.

Primary L'.\'aminer-R. L. Andrews Attorney. Agent, or FirmArmstrong, Nikaido & Wegner [57] ABSTRACT Method for continuous production of electrolytic manganese dioxide by electrolysis with an aqueous solution of manganese sulfate-sulfuric acid containing as impurity g/l or more of magnesium in the form of magnesium sulfate while making up for consumed manganese by addition of a neutral aqueous manganese sulfate solution to electrolyte, is improved by concentrating said neutral aqueous manganese sulfate solution under reduced pressure at C 60C to the volume corresponding to saturation point at from l5C to 60C and then cooling it to l5C 25C. Crystal thus precipitated which mainly consists ofl magnesium sulfate is filtered away and the filtrate is added to the electrolyte to compensate for the consumed manganese. During electrolysis the concentration of magnesium in the form of magnesium sulfate is maintained at g/l or less for avoiding troubles caused by the accumulation of magnesium sulfate in the electrolyte.

5 Claims, 2 Drawing Figures MnSO4+MgSO4-WATER EQUILIBRIUM 0 e w IOO- 3; 0

2 P so- I A c X B 260.

2 40- $5 C D s;

olbz'oiodoosbvboo PATENTEB AUG] 91975 s. I :1 a? g;

F l (5.2 MANGANESE ORE(POWDERED) LEACHING TANK MnOz OXIDATION TANK VACUUM EVAPORATDR COCOS NEuTRAL|zAT|0N TANK T VACUUM FILTER CRYSTALUZER SPENT ELECTROLYTE i NEUTRAL soLuTlo l CENTRIZFUGE T 6 I y '1 E TR MOTHER CRYSTAL YTIC CELL LIQUOR I ELECTROIXTIC MANGANESE DIOXIDE FINISHINGMASH- FEW DISCARDED ELECTROLYTIC MANGANESE DIOXIDE PRODUCT METHOD FOR CONTINUOUS PRODUCTION OF ELECTROLYTIC MANGANESE DIOXIDE The present invention relates to a method for producing electrolytic manganese dioxide, and more particularly to a method for continuous production of electrolytic manganese dioxide while preventing the accumulation of magnesium in the electrolyte, which derives from manganese ore.

In general, the electrolytic manganese dioxide is manufactured by preparing the electrolyte of aqueous manganese sulfate sulfuric acid solution which is prepared by extracting manganese ores such as manganese carbonate ore (Rhodochrosite) or reducing roasted manganese dioxide ore with aqueous solution of sulfuric acid and electrolyzing said electrolyte using insoluble electrodes such as lead, lead alloy, titanium, or graphite electrodes to deposit manganese dioxide on the anode. The over-all chemical reaction which takes place during the electrolysis is expressed by the following equation:

In the industrial application, it is advantageous to make up for the consumed manganese by supplying the leaching solution of the manganese ore to the electrolyte either continuously or intermittently to keep the concentration of manganese in the electrolyte substantially constant and to use overflowing electrolyte or spent electrolyte for the leaching of the manganese ore, sup lying sulfuric acid thereto, if necessary.

In a typical example of industrial application where electrolysis is carried out continuously. the electrolyte containing 40 g/l of manganese in the form of manganese sulfate and 100 g/l of sulfuric acid is used, to which manganese is supplied for making up for the consumed manganese. The manganese dioxide deposited on the anode under the conditions that the electrolyte temperature is 85 98C and the anode current density is 0.7 l.2 Amp/dm is stripped off every [0 to days. The making up for the consumed manganese is carried out by extracting manganese from ore with the spent electrolyte. The amount of manganese to be extracted is equal to that consumed during electrolysis. The extract is fed to electrolytic cell, whereas the same volume of spent electrolyte comes out of the cell.

One of the difficulties encountered in the above industrial method of manufacturing electrolytic manganese dioxide is that the impurities extracted together with manganese with the spent electrolyte are accumulated in electrolyte in the form of sulfate, which spoils the elctrolyte and degrades the quality of the product.

The manganese ore which contains 20 4O (by weight) of manganese, usually contains more or less impurities such as heavy metals e.g. iron, alkaline earth metals such as calcium, magnesium and alkali metals such as potassium. These impurities are extracted, during the leaching of the ore, together with manganese. The impurities, particulary iron, can be removed by oxidizing the leached solution with an oxidizing agent such as manganese dioxide. converting Fe ion to Fe ion, followed by neutralizing the solution by alkali such as lime to the extent of pH 4 6 and filtering away the resulting precipitate of iron hydroxide and calcium sulfate together with the leaching residue. In this process other impurities contained in small amount in the ore, such as arsenic, are absorbed on the precipitate and fil tcred away, but magnesium which is contained in a relatively large amount in the ore remains in the filtrate and cannot be removed fully. Accordingly, in the course of supplying the filtrate, i.e. neutral aqueous manganese sulfate solution, to the electrolytic cell. magnesium sulfate is accumulated in the electrolyte.

With the accumulation of magnesium sulfate in the electrolyte, the specific gravity and the viscosity of the electrolyte increase, impeding the diffusion of ions and causing the reduction of the electric conductivity, which in short disable uniform deposition of manganese dioxide. The tendency appears when the concentration of magnesium exceeds 30 g/l in the electrolyte, and when the concentration exceeds 45 g/l, magnesium sulfate crystallizes not only on the surface of the electrode but also on the liquid surface between the electrodes in piles and push out the electrodes and cause poor electrical contact. Furthermore, the crystallization of magnesium sulfate also takes place even in pipe in which the electrolyte flows, causing serious problems.

The usual way to avoid the above difficulties is to discard a part of the electrolyte in order to prevent the accumulation of magnesium sulfate in the electrolyte. This method, however, is not economical in view of the loss of manganese and the increase of consumption of sulfuric acid and lime which is used as a neturallizer of the discarding electrolyte. Another method has been proposed in which hydrofluoric acid is added to the electrolyte to precipitate magnesium as insoluble magnesium fluoride to be removed. The use of hydrofluoric acid, however, renders the method too expensive.

it is an object of the present invention to provide a method for continuous production of electrolytic manganese dioxide by an economical and simple process while preventing the accumulation of magnesium sulfate in the electrolyte, regardless of the amount of magnesium content in the raw material of manganese ore.

The present invention is based upon the discovery of the fact after an extensive study of the mutual solubility of manganese sulfate and magnesium sulfate in the mixed solution of the two, that magnesium can be continuously removed by concentrating the mixed solution of manganese sulfate and magnesium sulfate at a particular temperature and cooling it down to a particular temperature to thereby separate it into crystal primarily consisting of magnesium sulfate and mother liquor primarily consisting of manganese sulfate.

The present invention provide an improved method for continuous production of electrolytic manganese dioxide by electrolyzing an aqueous manganese sulfate sulfuric acid solution accompanied with 30 g/l or more of magnesium in the form of sulfate as an impurity, and making up for the consumed manganese by supplying electrolyte with neutral aqueous manganese sulfate solution which is prepared by leaching manganese ore containing magnesium as an impurity with spent electrolyte containing excessive amount of sulfuric acid and then neutralizing it, wherein all or a part of said neutral aqueous solution of manganese sulfate is concentrated under reduced pressure at 40C C to the volume corresponding to saturation point of from 15C to 60C, then cooled to l5 25C. the resulting crystal mainly consisting of magnesium sulfate is filtered away, and the mother liquor is added to the electrolyte to make up for the all or a part of the consumed manganese, whereby the content of magnesium in the form of magnesium sulfate in the electrolyte is maintained not more than 4 g/l.

Literatures show that in the ternary system of manga nese sulfate magnesium sulfate \Mlltil', the ratio of manganese to magnesium in saturated solution varies with the temperature. It is also known that the solubil ity is rather low at elevated temperature e.g. at )ll"(', and that the solubility is the highest in intermediate temperature range i.e. around SU"(, and that solid phase thereof comprises MgSll. ll t) l MnS() II. at higher temperature range, MnSt), H at the intermc diatc temperature range and MgSUJIlJ) at lower tern peraturc range (Soluhilities of Inorganic and Metal ()rganic ('ompounds" by Atherton Seidell, Vol I, page 987 i940), Published by I). Van Nostrand to.)

'l'he inventors of the present invention have coniirmcd that in the neutral aqueous manganese sulfate solution said behaviors of the suitability in the interme diate and the lower temperature ranges stood true over a wide range of proportion of manganese sulfate and magnesium sulfate and have found the method of re moving a part of magnesium from the neutral solution to be used as a feed solution to the electrolysis.

The present invention will he described in more detail with reference to the accompanying drawings, in

which:

FIG. I shows equilibrium curves of the ternary system, manganese sulfate 4 magnesium sulfate water, at and C.

l|(i. 2 shows a ilow chart illustrating an embodiment of the present invention.

Referring to FIG. I where the equilibrium curves of the ternary system, manganese sulfate magnesium sulfate water, at 50C and 20C are shown, the ordinate stands for the concentration of manganese and the ahsissa stands for the concentration of magnesium, and the curve I represents the equilibrium at 50C while the curve 2 represents that at Z(l(.

If the neutral aqueous solution having the composition at point X (Mn (til g/I, Mg 40 g/l) is concentrated at 50C, the concentration will change as X Y I 7. provided that the ratio of Mn to Mg is kept constant. At point I crystallization begins to take place. The crystal is MnSO H O. Since manganese sulfate is crystallized selectively the liquid is enriched with magnesium sulfate with continuing the concentration at that temperature. In the present inven tion, an end point of the concentration is set at point 7., and when the concentration is carried out to this point, the solution is then cooled to l5 C. If the concentrated solution is gradually cooled to 20C, for instance, the composition of the crystal deposited at each temperature between 50C and 20C varies depending upon the liquid solid equilibrium at that tem perature. However, if the cooling is carried out so rapidly that the transient over saturation takes place at the temperature between SU"(' and 2U"( and crystalliration takes place predominantly at 20C. the composi tion of crystal thus obtained can be rich in MgSO 7H If the concentration is carried out to any point P intermediate the points Y and 7., the amount of crys tal crystallived at 20C decreases but the ratio of magnesium to manganese in the crystal increases.

In practicing the present invention the concentration temperature is chosen between 4()"( and (0C. and preferably between 5()"( and 6U"(. 'I'he degree of concentration may be suitably determined by the coordina- (il l tion of the amount of magnesium to he removed and the amount of manganese to he lost, the use of ll(i. I, MnSO MgSU, H O lzquilihrium diagram. is conve nient for determining of the degree of concentration depending upon the composition of original solution. For example, if the concentration is to be carried out to the point 7. in FIG. I, the concentration ratio (voI ume of concentrated solution/volume of original solution) is given by (R ()7, X lt)(l/r. Similarly the concentration ratio at the point I can be given by (R )I ltlilfi 'l be concentrated solution is rapidly cooled to l5"( 25"(, preferably 2()"(' 23% to precipitate crystal composed mainly of magnesium sulfate.

'l'he present invention can be applied to the solution of any manganese concentration when the accumulation of magnesium in the electrolyte exceeds 30 g/l in the process of manufacturing manganese dioxide by electrolysis of aqueous manganese sulfate sulfuric acid solution and can be more advantageously applied to the electrolyte used in the industrial continuous process as stated hereinbefore which contains it! 40 g/l of manganese and 20 ltlll g/l of sulfuric acid in order to maintain the concentration of magnesium below 30 45 g/l, preferably below 30 g/l.

Industrial application of the present invention will now be described with reference to the llow chart shown in FIG. 2.

Pulverized manganese ore is fed in a leaching tank to be leached with the spent electrolyte from electrolytic cell. The amount of manganese in the ore to be fed is equal to that consumed during electrolysis and lost in the processes of leaching and purification. The spent electrolyte contains sulfuric acid sufficient to extract soluble components of the manganese ore as sulfate. Sulfuric acid is, therefore, added as required. The leached solution still exhibits acidic property and it is fed to an oxidation tank together with the leaching residue, where they are brought into contact with powder of manganese dioxide and Fe" in the leached solution is converted into be. In this process, either electro lytic or natural manganese dioxide can be used as an oxidizing agent. The solution is then delivered to a neutralization tank. In the neutralization tank, limestone is added to neutralize sulfuric acid in the leached solution to precipitate iron as ferric hydroxide. Final pH value in the neutralization step lies between 4 6. The neutralized solution is fed to a filter such as filter press to filter away the solid-leaching residue, calcium sulfate and the precipitate of ferric hydroxide.

The neutral aqueous manganese sulfate solution of thus prepared contains 40 g/l of manganese and 3t) 50 g/l of magnesium in the form of sulfate. The solution also contains calcium sulfate with its saturated concentration and small amount of potassium sulfate. The present invention is applied to this neutral aqueous solution.

In the practice of the present invention, either full volume ofor a part of the neutral aqueous solution may be subjected to a deanagnesium treatment. This is determined appropriately depending upon the amount of magnesium permissible to be present in the electrolyte. In general, the amount of magnesium contained in the manganese ore is less than that of manganese. The concentration of magnesium in the electrolyte can be maintained constant by removing magnesium which comes from the ore. hence the amount of solution subjected to the treatment is usually only a small portion as compared with the total amount of the ptocess solu tion. In this case the making up for the consumed man gauese is carried out liy adding hoth untreated solution and treated solution and treated solution [lUIlI which the crystal containing magnesium has been removed. In the dc magnesium trcatmcnL the ainotlnt ol' inanga nese lost in the precipitated crystal can lie made mini nmm hy a proper combination of the amount of aque ous solution to he processed and the concentration ratio to l'IC applied.

Referring to l"l( i. 2 a part ol' the neutral solution is led to a vacuum evaporator where it is concentrated at 4()"(' 5l)"( preferably at St)"( and then cooled to I5"(' prel crahly to .N)"(. The aimed concentra tion lies hetween the liquid composition at the satura tion point :it l5"( and that at (i()"('. For the neutral aqueous solution comprising ()(l g/l of Mn and g/l of Mg shown in l"l(iv I, l'or example. the minimum and maximum concentration ratios can he given lwy (R Y Hill "1 aml (TX 67. ltltV/ respectively. 'l'he composition corres mnding to the area hounded hy the leetangular AIKI) in FIG. I is the typical composition ol'the neutral aqueoils solution in the industrial contin nous process, and the concentration ratio for each composition can he determined in a similar manner.

Another advantage of this invention is that the intro duction olevaporation process into the electrolytic manganese dioxide manufacturing process makes it easy to control the total volume of process solution. llthe heating of the electrolyte in the electrolytic cell is carried out by direct injection of live steam instead of indirect heating with heat exchanging tuhes the vol ume ol the electrolyte increases because of condensation of steam in the electrolyte. Although the direct heating method has many advantages such as elimina tion ol'heating tubes, resulting in the reduction ol plant construction cost and of maintenance cost as well. the method is hardly applicable without a certain device such as evaporator which can eliminate excess water from the electrolyte. (oncentrating more amount of the neutral solution than the amount inst corrcspoml ing to the amount of magnesium to he removed. using a part ol'thc concentrated solution as it is lor supplying to the electrolytic cell and subjecting the other part to de-magnesium treatment, makes it possible to reduce the total volume of the neutral solution to he led for the purpose ol controlling the total volume ol'the electrolyte. The dotted lilies shown in FIG. 2 illustrate this ell'ect.

The crystal produced in a erystalliler is separated from mother liquor with a suitable separator such as centrifuge. The mother liquor is supplied, together with the remainder of the neutral aqueous solution. to the electroly. cell.

The electrolysis is carried out under the conditions set forth in accordance with the prior art method, and manganese dioxide deposited on the anode is stripped olT, washed and dried to provide linal product.

Several examples are given below to make the understanding ol the present invention clearer. although it is not intended to limit the present invention to any of the illustrated examples.

liXAMll .l l

2700 (trams of powdered manganese carbonate ore (consisting ol Mn 3 l .tWr Si( UV! l"e V7: (a 4% Mg 28% and K 0.1%. each represented by weight) was leached with 2l.2 l ol a solution prepared (the sulluric acid concentration. lli g/I) liy adding sull'uric acid to a spent electrolyte produced during electrolysis (spent electrolyte composition 215 g/l of Mn in lorm ol' man ganese sull'atc, $57 g/l of Mg in form of magnesium sulliitc and 82 l g/l of sulluric acid). l lILI operation temperature was kept at H91 Dissolved l-e" was oxi di/ed by adding It)! grams ol' manganese dioxide powder having a ale olnot more than lfil) mesh aml purity ol")5'/l. Powder of limestone was then added to render the pll value olthe solution to in. l'he neutralized so lution was then filtered to ohtain 202 l of aqueous sohr tion olmanganese snllate. l'he composition of the solution was Mn 59.0 g/l Mg .101) g/l a ()5 g/l and K ().2 g/l.

The whole volume ol the solution was put in a distillation flask and concentrated with at 81) mmllg (vac uum) at 5()"( to the extent that the volume ol'the solu tion is halved It) l). The composition of the concentrated solution contained Mn l2l g/l (Mn net content: l2lt) g) and Mg 78 g/l (Mg net content: 71') g) the concentration ratio being The concentrated solution was then cooled rapidly with a 20 nnnllg vacuum at 22"(' to precipitate crystal which was then separated into 9570 grams of crystal and 4.5? ol mother liquor by a centrifuge.

The mother liquor contained Mn I32 g/l (Mn net content: (105 g) and Mg 34.5 g/l (Mg net content: lfiti g) while the crystal contained Mn (3% (Mn net coir tent: (M5 g) and Mg (1.5% (Mg net content: (:23 g).

The experiment showed that by this method ol magnesium was removed while 50% of manganese was lost.

4.5% liters of the mother liquor (Mn I32 g/l. Mg 34.5 g/l) was then diluted with water to produce llLUXl ol feed solution (Mn ()0 g/l, Mg 15.7 g/l) lor the electrolysis, lileetrolysis was carried out by using titanium electrode as anode and graphite electrode as cathode, with Ll) A/dm" olanode current density at a lemperw ture ol )2"( for IS days. during electrolysis said leed solution was supplied at the rate ol'lHIZX l/hr. In order to maintain the electrolytic cell temperature at 92C. live steam was supplied at (L000 kg/hr. As a result, a spent electrolyte overflows at (UB2 l/hr (containing M11222 g/l. Mg 13.7 g/l. H. .S(), 54 g/l).

During the l5 day electrolysis, the cell voltage was kept at 2.5 2.(\ V and apparent current el'licieney was lllll'Z". The deposition of manganese dioxide was washed. duried to produce the product having a purity of 9571'.

Indirect heating with heating tuhe inserted into the electrolytic cell was applied and temperature was kept at J2"(. 'lhe overflowing rate of spent electrolyte was tl.ll25 l/hr and the composition thereof was Mn 28.4 g/l, Mg I71) g/l. H SU, (v9.5 g/l.

In the above electrolysis, when the neutral solution (Mn 5*)! g/l Mg PM) g/l) was not concentrated but used as feed solution, and live steam was applied at t).t)t)o kg/hr, the amount of the overflowing spent electrolyte was H.032 l/hr and the composition thereol' was Mn 228 g/l. Mg 34.2 g/I. H 50, 53 g/l. (ell voltage for l5 days was kept at 3.2 3.5 V and the current elliciency was ll-W1. Manganese dioxide deposited.

l-IXAMPLli 2 LS Liters of neutral manganese sull'ato solution obtained in the same manner as in the lixample l was concentrated under the same conditions as in Example l but the termination point of the concentration was shortened to abtain 6.25l of concentrated solution. The concentration ratio was. therefore. 65.8%. The composition of the concentrated solution in this case was Mn 90.8 g/l (Mn net content: 565 g) and Mg 59.3 g/l (Mg net content: 370 g). The concentrated solution was then cooled under the same conditions as in Example I to precipitate crystal to obtain 2870 grams of crystal (Mn 4.2% (Mn net content: I20 g). Mg 7.27r (Mg net content: 207 g)) and 4.35] of mother liquor (Mn 102.5 g/l, Mg 38 g/l).

This experiment showed that 56% of Mg was removed and Mn loss was as low as 21%.

4.35 Liters of mother liquor was then diluted with water to obtain 7.43l of feed solution (Mn 60 g/l. Mg 22.2 g/l) for the electrolysis. Electrolysis was carried out for l l days with supplying said feed solution under the same condition as in the Example I. Results of electrolysis were substantially the same as the Example 1 except that cell voltage was 2.5 2.7 V showing the tendency of rising somewhat at the end of electrolysis.

What is claimed is:

I. In a method for continuous production of electrolytic manganese dioxide by electrolysis of an aqueous manganese sulfate sulfuric acid solution containing 30 g/l or more of magnesium in the form of magnesium sulfate as impurity wherein consumed manganese is made up for by supplying a neutral aqueous manganese sulfate solution prepared by leaching of manganese ore which contains magnesium as impurity with spent electrolyte and sulfuric acid followed by neutralization and filtration;

the improvement comprising concentrating the whole amount of or a part of the neutral manganese sulfate solution in a vacuum at a temperature of between 40C. and 60C. to produce a solution having a concentration corresponding to a saturation point at from C. to 60C., and then rapidly cooling the resultant solution to l5C. 25C.. to give crystal containing magnesium sulfate, filtering away the crystal and feeding the filtrate to the electrolytic cell to make up for the whole amount of or a part of consumed manganese. whereby the con tent of magnesium in the form of magnesium sulfate in the electrolyte is maintained at not more than 45 g/l.

2. Method as defined by claim 1 wherein said spent electrolyte contains 10 40 g/l of manganese in the form of manganese sulfate and 30 45 g/l of magnesium in the form of magnesium sulfate.

3. Method as defined by claim 1 wherein said neutral aqueous manganese sulfate solution contains 40 g/l of manganese in the form of manganese sulfate and 30 50 g/l of magnesium in the form of magnesium sulfate.

4. Method as defined by claim 1 wherein electrolysis is carried out while blowing in live steam.

5. In a method for production of electrolytic manganese dioxide by electrolysis of an aqueous manganese sulfate sulfuric acid solution containing 10 40 g/l of manganese in the form of manganese sulfate and as impurity 30 45 g/l of magnesium in the form of magne sium sulfate, wherein consumed manganese is made up for by supplying a neutral manganese sulfate solution containing 40 70 g/l of manganese in the form of manganese sulfate and 30 50 g/l of magnesium in the form of magnesium sulfate, said neutral solution being prepared by leaching manganese ore containing magnesium as impurity with a spent electrolyte followed by neutralization and filtration;

the improvement comprising concentrating the whole amount of or a part of said neutral aqueous solution of manganese sulfate in a vacuum at a temperature between 40C. and 60C. to produce a solution having a concentration corresponding to a saturation point at from 15C. to 60C.. and then rapidly cooling the resultant solution to l5- 25C., to give crystal containing magnesium sulfate. filtering away the crystal and feeding the filtrate to the electrolytic tank to make up for the whole amount of or a part of consumed manganese. whereby the content of magnesium in the form of magnesium sulfate in the electrolyte is maintained at not more than 45 g/l. 

1. IN A METHOD FOR CONTINUOUS PRODUCTION OF ELECTROLYTIC MANGANESE DIOXIDE BY ELECTROLYSIS OF AN AQUEOUS MANGANESE SULFATE - SUFLURIC ACID SOLUTION CONTAINING 30 G/L OR MORE OF MAGANESIUM IN THE FORM OF MAGNESIUM SULFATE AS IMPURITY WHEREIN CONSUMED MANGANESE IS MADE UP FOR BY SUPPLYING A NEUTRAL AQUEOUS MANGANESE SULFATE SOLUTION PREPARED BY LEACHING OF MANGANESE ORE WHICH CONTAINS MAGNESIUM AS IMPURITY WITH SPENT ELECTROLYTE AND SULFURIC ACID FOLLOWED BY NEUTRALIZATION AND FILTRATION, THE IMPROVEMENT COMPRISING CONCENTRATING THE WHOLE AMOUNT OF OR A PART OF THE NEUTRAL MANGANESE SULFATE SOLUTION IN A VACUUM AT A TEMPERATUE OF BETWEEN 40*C. AND 60*C TO PRODUCE A SOLUTION HAVING A CONCENTRATION CORRESPONDING TO A SATURATION POINT AT FROM 15*C. TO 60*C., AND THEN RAPIDLY COOLING THE RESULTANT SOLUTION TO 15*C. - 25*C., TO GIVE CRYSTAL CONTAINING MAGNESIUM SULFATE, FILTERING AWAY THE CRYSTAL AND FEEDING THE FILTRATE TO THE ELECTROLYTIC CELL TO MAKE UP FOR THE WHOLE AMOUNT OF
 2. Method as defined by claim 1 wherein said spent electrolyte contains 10 - 40 g/l of manganese in the form of manganese sulfate and 30 - 45 g/l of magnesium in the form of magnesium sulfate.
 3. Method as defined by claim 1 wherein said neutral aqueous manganese sulfate solution contains 40 - 70 g/l of manganese in the form of manganese sulfate and 30 - 50 g/l of magnesium in the form of magnesium sulfate.
 4. Method as defined by claim 1 wherein electrolysis is carried out while blowing in live steam.
 5. In a method for production of electrolytic manganese dioxide by electrolysis of an aqueous manganese sulfate - sulfuric acid solution containing 10 - 40 g/l of manganese in the form of manganese sulfate and as impurity 30 - 45 g/l of magnesium in the form of magnesium sulfate, wherein consumed manganese is made up for by supplying a neutral manganese sulfate solution containing 40 - 70 g/l of manganese in the form of manganese sulfate and 30 - 50 g/l of magnesium in the form of magnesium sulfate, said neutral solution being prepared by leaching manganese ore containing magnesium as impurity with a spent electrolyte followed by neutralization and filtration; the improvement comprising concentrating the whole amount of or a part of said neutral aqueous solution of manganese sulfate in a vacuum at a temperature between 40*C. and 60*C. to produce a solution having a concentration corresponding to a saturation point at from 15*C. to 60*C., and then rapidly cooling the resultant solution to 15*- 25*C., to give crystal containing magnesium sulfate, filtering away the crystal and feeding the filtrate to the electrolytic taNk to make up for the whole amount of or a part of consumed manganese, whereby the content of magnesium in the form of magnesium sulfate in the electrolyte is maintained at not more than 45 g/l. 